1. Field of the Invention
This invention generally relates to integrated circuit (IC) fabrication and, more particularly, to an indium oxide (In2O3) film with a controlled resistivity for use in ferroelectric memory devices, and a method for fabricating the same.
2. Description of the Related Art
One-transistor (1T) ferroelectric memory devices are conventionally made with MFMOS (Metal, Ferroelectrics, Metal, Oxide and Silicon) or MFOS (Metal, Ferroelectrics, Oxide, and Silicon). The retention properties of 1T ferroelectric memories present a considerable technical challenge, due to the generation of a depolarization field once the device has been programmed. Three possible mechanisms may be responsible for the poor retention of 1T ferroelectric memories, namely: leakage current; trapped charge within the ferroelectric film; and, the depolarization field. Because the leakage current increases with increasing the temperature, the leakage current may seriously affect the retention properties at high temperatures. Trapped charges in the ferroelectric film are due to the high density of internal defects. Trapped charges, working together with leakage current, may also affect the retention property. The retention time (t) for a 1T device that has a remnant polarization (Pr), leakage current (I) and a trapping density (d) may be estimated to be t=Pr/Id. The formula shows that higher leakage current and trapping density result in poor retention properties.
On the other hand, for 1T MFMOS memory devices, the floating gate may be neutralized because of the leakage current, and the remnant polarization of ferroelectric materials cannot be applied to the channel. In this case, the memory function of 1T MFMOS devices will be lost. The depolarization field applied to the ferroelectric dielectric after programming is due to the existence of an oxide capacitor in series with the ferroelectric capacitor in both MFMOS and MFOS memory cells. The linear capacitor tends to discharge all stored charge and, therefore, generate a voltage opposed to the ferroelectric polarization, which can be expressed as QR/(CFE+COX), where QR is the remnant charge, CFE and COX, are the capacitances of the ferroelectric and oxide capacitors, respectively. The depolarization field, opposes the ferroelectric film polarization, and decreases the initial applied polarization so that, in time, the polarization reaches a steady state. The result is a 1T FE memory device with poor memory-retention properties, especially at higher temperatures. That is, the memory state of the device is retained for a relatively short life, on the order of one month.
It would be advantageous if a thin film could be developed with a controlled resistivity, for the replacement of a gate insulator material in a 1T FE memory device.